Place popcorn in a lightly greased large roasting pan; set aside. Combine butter, brown sugar, corn syrup and salt in a heavy saucepan. Bring to a boil over medium heat; cook 5 minutes. Remove from heat. Add vanilla and baking soda; stir well and pour over popcorn. Mix well.
Bake at 250° for one hour; stirring occasionally; let cool. Break into pieces and store in an airtight container. Makes 27 cups.